GML writer: fix missing SRS in Featurecollection's boundedBy element (3.9.1 regression)

[rouault]

Fixes #10332

Was caused by the fix 3674ec7570f324ea43eef08b6f5ce3a29de2741d for #10071

The regression mostly occurs when writing a GML file using ogr2ogr when the source layer has a named geometry column, e.g if the source is a GeoPackage or if using -sql The problem was acutally latent and also could have occured before 3.9.1 if using CreateLayer() without a SRS + CreateGeomField()